The present invention relates to a laser oscillator device, in particular to a laser oscillator device with improved cooling ability and cooling efficiency in the lasing medium.
The basic principle of laser oscillator devices is to illuminate solids such as ruby or gases such as carbon dioxide with an excitation beam to cause a high-energy inversion in their atoms, such that a resonator can be used to amplify the light emitted when their energy states return to their normal level, enabling the light to be extracted in the form of a phase-matched beam of a single color.
FIG. 1 shows the basic idea behind a laser oscillator device. For example, a lasing medium 110 such as ruby receives an excitation beam 120 from a xenon lamp, arc lamp or the like, with light amplification being performed on an optical axis 130 perpendicular to the excitation beam 120. The coherent laser beam 140 amplified and emitted from the lasing medium 110 propagates along the optical axis, is reflected by the mirror 150 on the right side of the drawing, passes in the opposite direction through the lasing medium 110 where it is further amplified, then reaches the mirror 155 on the left side of the drawing where it is reflected back toward the lasing medium 110. The laser beam 140 which is amplified by repeated reflections through the lasing medium 110 in this way can be directed outside the resonator by a combination of a polarized beam splitter 170 and a Pockels cell 160 for selectively rotating the polarization of the laser beam according to a control voltage, or a simple output coupling mirror.
In the drawings, the reference number 180 indicates a cooling device. Since the lasing medium 110 receives an excitation beam 120 and emits a laser beam 140, it can also generate intense heat during operation, so that in order to maintain the temperature of the lasing medium 11 to within an operable temperature, the heat usually must be removed by a cooling device.
While the directions of the excitation beam and the laser beam are perpendicular in the drawing, they may alternatively be provided on the same axis. Many different types of lasing media are known, including ruby, titanium sapphire, alexandrite, Nd-YAG, Er-YAG, dyes, diodes and carbon dioxide, these being potentially in solid, liquid or gaseous form.
The laser beam which is extracted on the above-described principles is highly monotone, coherent, highly directional, and high in energy density, so that it can be used in a very broad range of fields by taking advantage of the various properties. The anticipated types of application can be largely separated into four fields. The first field is microprocessing of optical communication parts. The second field is boring of holes into steel such as in the microinjectors of automobile engines or the like. The third field is medicine, such as treatment of portions where nerves or brain cells are concentrated around the illuminated areas such as in the brain or spine. The fourth field is new semiconductor devices, where they can be used to form fine periodic structures on silicon.
In particular, in most of the above fields, it would be desirable for the output of laser devices to become higher in the future from a variety of practical standpoints. This is because higher power would greatly improve the processing speed.
When performing laser oscillation, the laser oscillator device, including the lasing medium, must naturally be maintained within a predetermined operational temperature range, but if the output power of the laser device is made higher, particularly if the power density of the lasing medium is made higher, the lasing medium can become an extremely strong heat source, thus requiring a method and device capable of adequately and efficiently removing heat generated from the lasing media.
Furthermore, as the output power of the laser device becomes greater, the increase in temperature of the lasing medium is accompanied by a reduction in the thermal conductivity, thus reducing the cooling effect. Consequently, the temperature gradient in the lasing medium becomes very pronounced, generating high-temperature areas, and this effect is one of the factors restricting the power of the laser. There are two reasons for this. First, thermal lensing occurs. Thermal lensing refers to the effect wherein heat which is absorbed by the lasing medium and not eliminated heats up the laser crystal, resulting in a temperature distribution that gives rise to a corresponding refractive index distribution, causing the lasing medium to act as a lens. While thermal lensing due to an excitation laser of up to about 20 W can be handled by modifying the laser resonator, if the power is greater than this, the focal distance of the thermal lens becomes shorter than the laser crystal, thus making such measures useless. The second reason is the occurrence of birefringence due to thermal expansion. For example, birefringence will occur in a crystal which is not naturally birefringent such as a YAG base, given enough heat. This causes the polarization of the beam to be greatly disturbed on each pass through the crystal, resulting in a considerable loss at the polarizers in the resonator.
FIG. 2 is a drawing schematically illustrating a conventional device used for cooling a lasing medium. The lasing medium 210 is housed in a jacket 220 of copper or the like which functions as a heat sink, and one of the outer surfaces 230 of the jacket 220 is cooled by supplying with liquid nitrogen 250 that is stored in a tank 240. If the thermal density of the laser medium is less than or equal to a certain level, then even cooling devices as shown in FIG. 2 based on the conventional art will be able to perform cooling without any functional obstacles, apart from the fact that the equipment becomes quite bulky. However, if the power becomes greater, the following problem can occur.
That is, when the jacket is cooled by contact with liquid nitrogen, the liquid nitrogen will vaporize and form bubbles if the thermal density becomes very large, but the bubbles will reduce the area of effective contact with the liquid nitrogen and consequently result in reduced cooling ability. More specifically, as long as the heat-carrying medium is liquid, the cooling efficiency will be high because the heat capacity per unit volume is higher than that of a gas, but if the heat-carrying medium is vaporized by the heat from the lasing medium, a flow consisting of a liquid and a gaseous phase will be generated. This results in non-linearities between the heat capacity and amount of heat removed, as well as making it difficult to control the temperature of the lasing medium due to a drop in the cooling ability as described above.
Furthermore, the increased heat capacity from the laser medium will require the liquid nitrogen tank to be made larger, and when considering the need to provide liquid nitrogen replenishing equipment and equipment to vent off the vaporized nitrogen, the equipment will need to be made even larger.
Although it is effective to cool the laser medium to a lower temperature than is conventional in order to reduce the temperature gradient occurring in the laser medium to raise the output power, the liquid nitrogen that is conventionally used has an evaporation temperature of about 77.3 K at 1 atmosphere, so that it is theoretically impossible to cool it to a temperature lower than this. While lower temperatures can be achieved by using liquid helium which has an evaporation temperature of 4.2 K as the medium, the problem of the reduced cooling ability due to vaporization of the liquid nitrogen at the thermal exchange interface is still not resolved as in the case where liquid nitrogen is used. Furthermore, the equipment required for insulated containers and the like must be made even larger than in the case of liquid nitrogen when using helium which has a lower evaporation temperature.